The present invention relates to moisture detectors for automotive vehicle windshields, and more particularly to control systems incorporating such sensors to operate vehicle accessories such as windshield wipers.
A wide variety of windshield moisture sensors have been developed for controlling the actuation of an automotive vehicle windshield wiper and/or washer. Generally speaking, the sensors are of three types--optical, capacitive, or resistive. In optical systems, a light beam is directed into the windshield at an angle; and the refracted/reflected portion of the beam is monitored to evaluate whether moisture is present In capacitive sensors, two electrodes are mounted on the windshield surface; and the capacitance between the electrodes is monitored to determine the presence of moisture. In resistive systems, the resistance encountered by the wiper motor in driving the wipers across the windshield is monitored; and the wipers are stopped when the resistance reaches a preselected drag level indicating that the windshield is dry.
The present invention relates to optical technology, which suffers at least three significant disadvantages. First, the optical detectors in such systems are rather sensitive to ambient light which often contains a large amount of infrared light. Headlights, taillights, commercial signs, and other filament-type light bulbs are significant sources of IR light. Not infrequently, ambient light saturates the control system and/or reduces the amplitude of the modulated useable IR control signal, triggering "false wipes" and/or making detection of the modulated infrared signal nearly impossible. An example of such a system is illustrated in U.S. patent application Ser. No. 073,496, filed July 15, 1987, by Hochstein, and entitled ELECTRO-OPTICAL WINDSHIELD MOISTURE SENSING NOW U.S. Pat. No. 4,798,956.
A second problem is that the fixed reference signal, even when user-selectable, used in such systems provides inadequate performance required by present day commercial applications. As noted above, the infrared portion of ambient light can trigger "false wipes". Further, the system components degrade with age and the performance varies with temperature and thereby results in a system where false wipes are continuous or no wipes will occur even when moisture is on the windshield. Examples of such systems are illustrated in U.S. Pat. No. 4,481,450 issued Nov. 6, 1984 to Watanabe et al entitled SYSTEM FOR CONTROLLING A VEHICLE WINDOW AND THE LIKE. Although the fixed reference signal can be altered, such systems are still subject to the variables of ambient light, temperature, and subsequent component degradation.
A third problem occurs concerning the interface of the conventional wiper control switch and the rain sensor control. For example, in a first type of interface, the system is switchable between rain-sensor control and conventional control regardless of the conventional control setting. Such a system is illustrated in British Patent No. 1,101,441 issued Jan. 31, 1968 to Kempston Electrical Co., Ltd. and entitled IMPROVEMENTS IN WINDSCREEN WIPERS. This prohibits continuous operation of the wipers when the system has been switched to the rain-sensor mode. In a second type of interface, the conventional switch is modified to replace the "intermittent" mode with a "rain sensor" mode so that rain-sensor control is always substituted for the intermittent mode. Examples of such systems are illustrated in U.S. Pat. Nos. 4,689,536 issued Aug. 25, 1987 to Iyoda entitled APPARATUS AND A METHOD FOR CONTROLLING A WINDSHIELD WIPER; 4,542,325 issued Sept. 17, 1985 to Kobayashi et al entitled RAIN CONDITION DEPENDENT WIPER CONTROL SYSTEM FOR AN AUTOMOTIVE VEHICLE; and 4,317,073 and 4,131,834 issued Feb. 23, 1982 and Dec. 26, 1978, respectively, to Blaszkowski entitled WINDSHIELD WIPER CONTROL SYSTEM. These systems prevent operation of the wipers in the intermittent mode, which is desired by many drivers.